1. Field of the Invention
The present invention relates to an apparatus for controlling the angular displacement of a throttle valve in response to the operation of an accelerator, in which a throttle valve is opened or closed by means of a motor in response to operation of the accelerator.
2. Description of the Prior Art
Conventional throttle valve angular displacement control systems are exemplified in a Japanese Patent Examined Open No. Sho. 58-25853 and Japanese Patent Unexamined OPen No. Sho. 59-58131.
Both Japanese Patent documents disclose an actuator comprising a reversible step motor which is connected to a throttle valve located within an intake air passage (throttle chamber) of an engine mounted on a vehicle and which actuates the throttle valve to open or close according to an electrical pulse signal sent to the actuator.
However, in cases where the opening angle of the throttle valve is estimated from a number of step pulses sent to the step motor in the conventional apparata described above, the positional accuracy of the throttle valve is determined by the resolution of the pulses. Therefore, fine adjustment of the opening angle of the throttle valve is difficult even given a higher accuracy than the current accuracy. In addition, since the opening angle of the throttle valve is detected by means of an electrical potentiometer and the detected opening angle is used as a feedback value, additional errors result in the control of the opening angle of the throttle valve due to deviations of the mounting of the potentiometer or fluctuations in a power supply voltage of the potentiometer.
Consequently, it may occur that the accelerator is ordered to a position corresponding to a fully closed state of the throttle valve, but the conventional control apparata cannot accurately hold the throttle valve at the fully closed position so that the engine idling speed may unnecessarily increase and/or the throttle valve may repeatedly abut the walls of the intake air passage at high speed, causing undue wear.